1. Field of Invention
This invention relates to xerography and more particularly to an improved method for the development of an electrostatic image in which a toner layer is presented to a latent image for its development.
2. Description of Prior Art
In the xerographic reproduction process, a photoconductive surface is charged and then exposed to a light pattern of the information to be reproduced, thereby forming an electrostatic latent image on the photoconductive surface. Toner particles, which may be finely divided, pigmented, resinous material are presented to the latent image where they are attracted to the photoconductive surface. The toner image can be fixed and made permanent on the photoconductive surface or it can be transferred to another surface where it is fixed.
One known method of developing latent electrostatic images is by a process called transfer development. Transfer development broadly involves bringing a layer of toner to an imaged photoconductor where toner particles are transferred from the layer to the imaged areas. In one transfer development technique, the layer of toner particles is applied to a donor member which is capable of retaining the particles on its surface and then the donor member is brought into close proximinity to the surface of the photoconductor. In the closely spaced position, particles of toner in the toner layer on the donor member are attracted to the photoconductor by the electrostatic charge on the photoconductor so that development takes place. In this technique the toner particles must traverse an air gap to reach the imaged regions of the photoconductor. In two other transfer techinques the toner-laden donor actually contacts the image photoreceptor and no air gap is involved. In one such technique the toner-laden donor is rolled in non-slip relationship into and out of contact with the electrostatic latent image to develop the image in the single rapid step. In another such technique, the toner-laden donor is skidded across the xerographic surface. Skidding the toner by as much as the width of the thinnest line will double the amount of toner available for development of a line which is perpendicular to the skid direction, and the amount of skidding can be increased to achieve greater density or greater area coverage.
It is to be noted, therefore, that the term "transfer development" is generic to development techniques where (1) the toner layer is out of contact with the imaged photoconductor and the toner particles must traverse an air gap to effect development (2) the toner layer is brought into rolling contact with the imaged photoconductor to effect development, and (3) the toner layer is brought into contact with the imaged photoconductor and skidded across the imaged surface to effect development. Transfer development has also come to be known as "touchdown development".
In a typical transfer development system, a cylindrical or endless donor member is rotated so that its surface can be presented to the moving surface of a photoconductive drum bearing an electrostatic latent image thereon. Positioned about the periphery of the donor member are a number of processing stations including a donor loading station, at which toner is retained on the donor member surface; an agglomerate removal station at which toner agglomerates are removed from the toner layer retained on the surface of the donor member; a charging station at which a uniform charge is placed on the particles of the toner retained on the donor surface; a clean-up station at which the toner layer is converted into one of uniform thickness and at which any toner agglomerate not removed by the agglomerate removal station are removed; a development station at which the toner particles are presented to the imaged photoconductor for image development; and a cleaning station at which a neutralizing charge is placed upon the residual toner particles and at which a cleaning member removes residual toner from the peripheral surface of the donor. In this manner, a more or less continuous development process is carried out.
Among the typical donor members employed in the process heretofore was a metal cylinder covered with an insulating enamel upon which was coated a metal electrode in a gravure-screen pattern. A potential of up to 300 volts is impressed between the electrode and cylinder while the cylinder is rotated in a vibrating tray of toner powder. In a mass of toner that appears to be electrically neutral there will be roughly equal amounts of positively and negatively charged particles. Microsized electrostatic fields formed between the electrode and the cylinder cause toner of one polarity to deposit on the electrode and toner of the opposite polarity to deposit on the squares in the electrode. Clumps of excess toner are vacuumed off and the remaining uniformly thick toner layer is corona charged to make it all the same polarity, thus making the donor ready for use in developing an image.
As discussed previously the latent image on a photoconductive surface could be developed by momentarily "touching down" the donor member to the surface. The surface of the photoconductor containing the latent image is charged at a greater potential than the donor surface. Therefore, in charged areas of the surface, toner is attracted from the donor to the surface, in uncharged areas the toner-charge image forces keep the toner particles attracted to the donor, and the surface remains free of toner particles. However, it was found that several such "touchdowns" were needed to produce high density images because of a sparse migration of toner particles from the donor to the photoconductive surface.
Thicker coatings of toner produced by various techniques were explored in attempts to obtain the density desired with one touchdown, but these all seemed subect to the difficulty that where the thick coating of toner touched uncharged areas of, the photoconductor surface, some surface toner particles less strongly attracted to the donor transferred to the photoconductor producing an objectionable background deposit. The obvious solution was to bring the donor only very close to the photoconductor but not into contact with it. Toner will jump across a narrow air gap to charged areas of a xerographic photoconductor surface, but not to uncharged areas. The images thus obtained were greatly improved. This latter process has been termed "spaced touchdown".
It has also been found that the quality of image development can be further enhanced if a toner particle is repelled from the donor surface when the particle comes into the reach of an electrostatic flux line emanating from the image charge on the photoconductor surface. In this case, it can home in on the field line and thus develop the latent image. At the same time, if the proper charge relationship is established between the toner particles and the image and background charges on the photoconductor surface, toner should not move to areas of background on the photoconductor.
For example, U.S. Pat. No. 3,257,223 discloses a powder cloud xerographic development apparatus in which an aerosol of toner particles are formed adjacent to a photoconductor surface by removing the positive potential holding the particles to the donor member to create an unstable condition on the surface of the donor because of the great number of closely adjacent toner particles all having the same charge polarity. Owing to the mutual repulsion of these particles, many of the particles are rapidly forced away or "blown off" from the surface of the donor thus forming an aerosol in the space between the donor and the photoconductor surface being developed. Charged particles in this aerosol are picked up by the electric fields set up by the charge pattern on the photoconductor, thus serving to develop or make visible the charge pattern with these toner particles. The patentee also discloses that repulsion of the toner particles from the surface of the donor may be improved by connecting its conductive base to a potential source opposite in polarity to that utilized during the loading step rather than merely grounding this base. In this manner the repulsive force of a field emanating from donor member is added to the force of mutual repulsion between the toner particles thereby propelling them into the aerosol with greater velocity and uniformity.
The present invention expands and improves on this concept with application to both "in-contact" and "spaced touchdown development" xerographic apparatus.